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Optical and electrical characterization of low temperature grown GaInAs
This content has been downloaded from IOPscience. Please scroll down to see the full text. 1998 Semicond. Sci. Technol. 13 1031 (http://iopscience.iop.org/0268-1242/13/9/011) View the table of contents for this issue, or go to the journal homepage for more
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Semicond. Sci. Technol. 13 (1998) 1031–1035. Printed in the UK
PII: S0268-1242(98)90672-1
Optical and electrical characterization of low temperature grown GaInAs K Khirouni†, S Alaya‡, J Nagle§ and J C Bourgoink¶ † Laboratoire des Semiconducteurs, Faculte´ des Sciences, Route de Kairouan, 5000 Monastir, Tunisia ` Route de Medenine, ´ ` Tunisia ‡ Faculte´ des Sciences de Gabes, 6029 Gabes, § Laboratoire Central de Recherche, Thomson, Domaine de Corbeville, ´ 91404 Orsay Cedex, France ´ ´ et Het ´ erog ´ ` k Laboratoire des Milieux Desordonn es enes, Universite´ Pierre et Marie Curie, CNRS, Tour 13, 4 place Jussieu, ´ 75252 Paris Cedex 05, France Received 12 January 1998, accepted for publication 8 June 1998 Abstract. GaInAs grown at low temperature by molecular beam epitaxy has been studied using Hall effect and photoluminescence spectroscopy. The variations of the electrical and photoluminescence properties are described versus growth temperature and subsequent heat treatments. In contrast to GaAs, these materials exhibit a strong luminescence near band gap and are not semi-insulating. Recombination mechanisms are proposed from the dependence on the temperature and excitation density of the observed emissions, and reasons for their electrical behaviour are discussed.
1. Introduction Gallium arsenide epitaxial layers grown at low temperature by molecular beam epitaxy have recently attracted attention because of their potential applications, based on the homogeneity of their semi-insulating property and their very short lifetime. These materials are non stoichiometric with approximately 1% As in excess. They have a correct crystalline quality and, because they contain a high concentration of defects, they exhibit a poor luminescence [1–3]. The nature of the defects responsible for the electrical properties of these materials is still largely unknown. For instance, the electronic conduction can be related to hopping between defects [4, 5] or to the presence of metallic precipitates acting as buried Schottky barriers [6]. Although low temperature grown (LTG) GaAs has been studied systematically, this is not the case for other binary and ternary compounds. The realization of undoped epitaxial semi-insulating InP layers and of related alloys has not been achieved. The electrical properties are determined by shallow impurities or defects, and doping by transition metal impurities must be used to obtain a high resistivity. There have been only a few attempts to grow InP and GaP layers and alloys such as GaInAs, GaAlAs and AlInAs [7–12] at low temperature. There are only reports concerning their structural, electrical and optical properties [11–17]. GaInAs layers have been grown on InP substrates for temperatures ranging from 125 to 600 ◦ C. These layers ¶ E-mail address:
[email protected] c 1998 IOP Publishing Ltd 0268-1242/98/091031+05$19.50
are polycrystalline or highly disordered, which is attributed to a large As excess, of the order of 2%. Preliminary electrical measurements [17] suggest that the layers contain large concentrations of shallow levels but no midgap level. Annealing treatments decrease slightly the carrier concentration and increase the mobility. Similar results are obtained in layers grown on GaAs substrates [18]. Because the layers contain As precipitates, as LTG GaAs layers, it has been suggested that the drastic difference in the electrical behaviour must be ascribed to In. While As antisites exist and give rise to a midgap level in GaAs, these antisites could be suppressed or replaced by In antisites exhibiting very different electronic characteristics. The aim of this communication is to describe electrical and photoluminescence properties of LTG GaInAs layers and their correlation with the growth temperature and annealing treatments in order to understand the role of defects on the properties of this material. We shall see that the data support the view that As antisites do exist, characterized by a level at about 100 meV below the conduction band. 2. Experiment Samples of thickness 0.7 to 2 µm have been grown between 125 and 250 ◦ C (see table 1) by molecular beam epitaxy in a Varian reactor, on undoped InP (001) oriented substrates. Because the substrates are not in direct contact with the thermocouple, the growth temperature is not accurately known. However, the measured temperature corresponds to reproducible growth conditions for a given reactor. After 1031
K Khirouni et al Table 1. Conditions of growth of the studied layers.
Sample
Growth temperature
Thickness (µm)
Annealing temperature ( ◦ C)
a b c d e f g
125 150 150 200 250 250 226
0.74 1.08 1.88 2.35 2.05 2.05 2.29
— — 500 — — 500 —
Figure 2. Photoluminescence spectrum of sample e (250 ◦ C) as a function of the excitation density: 0.25 W cm−2 (dashed line) and 25 W cm−2 (full line).
Figure 1. Photoluminescence spectra of the layers grown at 250 ◦ C (a), 150 ◦ C (b) and 125 ◦ C (c) for an excitation density of 25 W cm−2 .
desorption under As pressure, the growth begins at 500 ◦ C by a 50 nm thick GaInAs layer, after which the temperature is lowered to start the growth of the LTG layer. The temperature of the substrate has been varied between 125 and 250 ◦ C, and a growth rate of the order of 0.5 µm h−1 was used. Some of the samples have been annealed in situ, for 15 min at 500 ◦ C under As pressure. For Hall effect measurements, ohmic contacts, typically 5 mm wide, are deposited on the top of the layers made by In allowing at 180 ◦ C for 10 min. The measurements are performed in the range 10 to 300 K. For photoluminescence measurements, the samples have been mounted on the cold finger of a closed cycle helium circulation cryostat in which the temperature varied from 9 to 300 K. Optical excitation is performed with the 514.5 nm line of an argon laser. The excitation power density, in the range 0.25–25 W cm−2 , is varied by a set of calibrated neutral density filters. The emission is analysed by a 0.6 m double monochromator and detected by a liquid nitrogen cooled germanium detector. 3. Photoluminescence results Typical photoluminescence spectra of as-grown layers are shown in figure 1. The spectrum of a sample (grown at 250 ◦ C) consists of a dominant peak labelled B, at 0.774 eV, and of a peak, A, at 0.800 eV. Peak B shifts slowly toward lower energies with excitation density and with temperature, as illustrated in 1032
Figure 3. Photoluminescence spectra of sample e (250 ◦ C) recorded at different temperatures showing the disappearance of peak B at high enough temperature.
figures 2 and 3. This behaviour is characteristic of a donor–acceptor recombination. However, its disappearance at high temperature confirms such an attribution since the appearance of an electron–acceptor emission is expected when the ionization of donors occurs. The second peak, A, shifts to higher energy (5 meV per decade) with increasing excitation density and is temperature dependent as illustrated in figure 4. In this figure, we recognize the S shape at low temperature observed in other ternary compounds which is attributed to the exciton ionization [19]. The plot of the peak intensity, in a logarithmic scale, versus T −1 (see figure 5) yields an activation energy of ∼20 meV. As we shall see in the next section, the resistivity of this layer is characterized by an activation energy, associated with electron ionization, of 10–20 meV. It is then reasonable to ascribe peak B emission to a defect located at about 15 meV below the conduction band. As illustrated in figure 6, peak A disappears after a post-growth annealing and a new peak C at 0.819 eV appears. Peak C, which does not evolve with the
Characterization of low temperature grown GaInAs
Figure 4. Variation of the energy of peak A versus the temperature of measurement in sample e.
Figure 7. Change of spectrum of sample f as a function of the excitation density: 2.5 W cm−2 (dashed line) and 0.25 W cm−2 (full line).
Figure 5. Variation of the photoluminescence intensity of peak A in the sample versus the inverse of the measurement temperature.
Figure 8. Effect of annealing on a layer grown at 150 ◦ C. Sample c annealed (dashed line) and sample b unannealed (full line).
Figure 6. Effect of annealing on the luminescence spectrum of a layer grown at 250 ◦ C. Full line: unannealed (sample e); dashed line: annealed (sample f).
is identical. From the behaviour of its intensity versus excitation density, this peak must be ascribed to a bound exciton. From its energy it becomes possible to deduce the alloy composition [20]: x = 0.48, which is in agreement with x-ray double diffraction measurements. Ibbetson et al [13] observed that LTG GaInAs layer contains precipitates which appear after a 500 ◦ C anneal. Since peak B disappears after such an anneal and is absent when the growth is performed at a conventional (high) temperature, it is tempting to ascribe this peak to the presence of radiative defects associated with As in excess. Finally, the photoluminescence spectrum of sample a is composed of a broad peak D centred at 0.848 eV (see figure 1). Observations by x-rays show that the crystalline quality of this layer is poor and it contains a large excess of As. 4. Hall effect
excitation density (see figure 7), seems to correspond to the photoluminescence spectrum of the materials grown at a lower temperature (see figure 8) if the In composition
The dependence of the resistivity of the layers versus the growth temperature is given in figure 9. It shows that this resistivity increases with increasing growth temperature 1033
K Khirouni et al
Figure 9. Variation of the layer resistivity versus the growth temperature. The open circles correspond to layers annealed at 500 ◦ C under As pressure.
Figure 11. Temperature dependence of the electron mobility in the same layers as in figure 10.
Figure 10. Temperature dependence of the free electron concentration in layers g (), d ( ), f (N) and a (✚).
Figure 12. Variation of the conductance versus frequency in layer e, measured at 8 (O), 15 ( ), 30 (✚) and 40 (N) K.
•
Table 2. Energetical locations of the defect levels detected by Hall effect measurements.
Sample
E1 (meV)
E2 (meV)
E3 (meV)
g d c f a
— — — 2 —
10 — — 10 12
110 80 105 — —
in the range investigated. It also indicates that a further thermal treatment at 500 ◦ C can have a drastic, or no effect depending on the growth temperature. The variation of this resistivity versus the temperature of measurements indicates that it is thermally activated above typically 200 K and practically constant below 50 K. Hall effect measurements show that the layers are n-type. From the variation of logarithm of the free electron concentration versus the inverse of the temperature (see figure 10), it appears that these electrons originate from three distinct levels, as shown in table 2. 1034
◦
Depending on the growth temperature, the electron mobility remains practically constant or increases slightly with the temperature of measurement (see figure 11). This indicates that this mobility is dominated by charged defects. This is also confirmed by the frequency dependence of the conductance which demonstrates that conduction occurs by hopping at low temperature, i.e. in a partially filled defect band. Indeed, like in LTG InP [21] and GaAs [22], the conduction varies as wx with x ∼ 1 for high enough frequencies w, regime which is temperature independent (see figure 12). In summary, the resistivity of the layers is low, of the order of 10−3 to 1 cm at room temperature except for the layers grown at the highest temperature. It is ∼100 cm for a growth at 250 ◦ C but an annealing treatment decreases it drastically. The conduction is induced by thermal ionization of electrons from shallow levels. The shallowest level corresponds probably to an hydrogenic donor impurity. The two deeper levels are still rather shallow thus preventing the material from being insulating.
Characterization of low temperature grown GaInAs
to the introduction of non-recombination centres which could be reasonably associated with the formation of As precipitates. Indeed, for the temperatures used in these annealing treatments, As is in excess and only interstitials are expected to diffuse. In conclusion, LTG GaInAs layers are not a material suitable for the production of highly insulating layers because the defects they contain do not exhibit the required properties. References
Figure 13. Band structure of the GaInAs alloys showing the location of the EL2 level ( ) and its extrapolation (dashed line). X is the Ga content.
•
5. Discussion and conclusion Low temperature grown GaInAs layers appear to contain only two electronically active donor defects whose associated levels are rather shallow. It is tempting to identify this defect with the As antisite since it is the dominant defect in LTG-GaAs. The reason the associated level does not lie at midgap, as in the case of GaAs, can be understood in terms of a band structure effect. Indeed, consider figure 13 which gives the band structure of the GaInAs alloys. It indicates the location of the midgap EL2 level associated with the As antisite. The position of this level has been studied by deep level transient spectroscopy as a function of the alloy composition [23]. The data are limited to a Ga composition x ranging from 0.8 to 1 and we extrapolate them down to x ∼ 0.5, the alloy composition of the studied layers. As illustrated in figure 13, we therefore expect the EL2 level to lie 150–200 meV below the conduction band. This value is larger than that of the detected level. However, because its concentration is large the defects are in interaction which result in the formation of a band, as evidence by the existence of the hopping regime of conduction at low temperature and the value of the energy level is reduced by the width of this band. Hence it is reasonable to conclude that the observed defect level is at the energy expected for the EL2 level, owing to the alloy composition considered and its concentration. The effect of annealing treatments which modifies the luminescence spectrum and its intensity must be ascribed
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